Reintroduce locking around socket writes

when transmitting multiple frames at once.

While changes in #350, #354 are supposed to be safe, there's some
evidence (#681) that sometimes they are not which leads to incorrect
byte stream interleaving on the wire.

Update API approval test expectations

Re-introduce lock for all socket writes

The official `NetworkStream` docs are confusing:

https://docs.microsoft.com/en-us/dotnet/api/system.net.sockets.networkstream?view=netframework-4.5.1

> Read and write operations can be performed simultaneously on an
instance of the NetworkStream class without the need for
synchronization. As long as there is one unique thread for the write
operations and one unique thread for the read operations, there will be
no cross-interference between read and write threads and no
synchronization is required.

This is a poorly-written way to say "multiple threads must be synchronized".

Fixes #681

Update API approval test expectations

projects/client/RabbitMQ.Client/src/client/impl/Connection.cs

@@ -254,6 +254,7 @@ public event EventHandler<ConnectionRecoveryErrorEventArgs> ConnectionRecoveryEr
                 }
             }
         }
+
         public string ClientProvidedName { get; private set; }
 
         public ushort ChannelMax

projects/client/RabbitMQ.Client/src/client/impl/SocketFrameHandler.cs

@@ -79,9 +79,8 @@ public class SocketFrameHandler : IFrameHandler
         private readonly ITcpClient m_socket;
         private readonly NetworkBinaryWriter m_writer;
         private readonly object _semaphore = new object();
-        private readonly object _sslStreamLock = new object();
+        private readonly object _streamLock = new object();
         private bool _closed;
-        private bool _ssl = false;
         public SocketFrameHandler(AmqpTcpEndpoint endpoint,
             Func<AddressFamily, ITcpClient> socketFactory,
             TimeSpan connectionTimeout, TimeSpan readTimeout, TimeSpan writeTimeout)
@@ -112,7 +111,6 @@ public SocketFrameHandler(AmqpTcpEndpoint endpoint,
                 try
                 {
                     netstream = SslHelper.TcpUpgrade(netstream, endpoint.Ssl);
-                    _ssl = true;
                 }
                 catch (Exception)
                 {
@@ -249,14 +247,7 @@ public void WriteFrameSet(IList<OutboundFrame> frames)
 
         private void Write(ArraySegment<byte> bufferSegment)
         {
-            if(_ssl)
-            {
-                lock (_sslStreamLock)
-                {
-                    m_writer.Write(bufferSegment.Array, bufferSegment.Offset, bufferSegment.Count);
-                }
-            }
-            else
+            lock (_streamLock)
             {
                 m_writer.Write(bufferSegment.Array, bufferSegment.Offset, bufferSegment.Count);
             }